Energy harvesters may be used to convert mechanical energy to electrical energy. The harvesting system typically includes a component that vibrates in response to mechanical energy passed to the harvesting system. For example, the harvesting system may be attached to a motor or other piece of equipment which vibrates. The conversion of mechanical energy to electrical energy may be accomplished using piezoelectric devices, capacitor devices or magnetic devices. In these systems, the most efficient conversion of energy from mechanical to electrical occurs when the resonant frequency corresponds to the frequency of the received vibration. In general, the conversion of mechanical energy to electrical energy may be quantified using the following equation:
          P        =            m      ⁢                          ⁢              ξ        e            ⁢                                    ω            3                    ⁡                      (                          ω                              ω                n                                      )                          2            ⁢              Y        2                                      (                      2            ⁢                          ζ              τ                        ⁢                          ω                              ω                n                                              )                2            +                        (                      1            -                                          (                                  ω                                      ω                    n                                                  )                            2                                )                2            wherein
P is the power that may be harvested,
m is the mass of the vibrating component,
ξe is the electrical damping ratio,
ω is the excitation frequency,
ωn is the natural frequency of the harvesting system,
Y is the amplitude of external vibration, and
ζτ is the total damping ratio.
Thus, the harvested power is maximized when the ratio of the excitation frequency to the natural frequency of the system approaches 1.
Harvesting systems that are to be used to harvest energy from a source that vibrates at a single dominant frequency may include a resonator tuned to the particular dominant frequency. This may be accomplished, for example, by adding a mass to a spring or lever arm so as to modify the resonant frequency of the spring or lever arm. For systems that vibrate at various discrete frequencies, the resonator used in the harvesting system may include different springs or levers tuned to the various frequencies or a number of different resonators, each tuned to a different frequency.
One can also employ devices which incorporate active components that are used to tune the resonant frequency of the resonator to the frequency then experienced by the device. Active components, however, require some amount of the harvested energy to be consumed, thereby reducing the effective output of the harvesting system.
Accordingly, it would be advantageous to provide a mechanical energy harvesting system with a tunable resonator. It would be further advantageous if harvested energy was not required to tune the resonator.